Chrysotile asbestos has long been used as the separator material for alkaline water electrolysis. However, for known health risk reasons and quality fall-off of the asbestos raw materials, asbestos separators are being abandoned. However, suitable alternatives are very scarce and still exhibit some drawbacks mainly related to their chemical stability in strong alkaline medium at high temperature (above 80° C.), and to their limited wettability.
Two approaches have been followed to realize ion-permeable diaphragms, both relying upon phase inversion of a binder-containing dope to provide the necessary porosity.
GB 1 270 133A discloses a method of applying liquid coating compositions simultaneously to both surfaces of a travelling web of indefinite length wherein the web is passed continuously between two opposed extrusion/doctor coating devices each having at least one doctor surface and at least one coating composition is fed continuously at constant pressure to each doctor surface to form at least one coating by passage between the surface of the web or a coating thereon and the respective doctor surface, the liquid supply pressures of the said coating compositions being so balanced that the web is maintained in a stable position intermediate the opposed doctor surfaces.
U.S. Pat. No. 5,650,011 discloses a duplex type coating apparatus for applying a coating liquid to a web, such as an elongated fabric, a plastic sheet and a metal sheet, comprising: a pair of dies arranged at opposite sides of a path of travel of the web while said web moves at a specified rate: a pair of coating liquid reservoirs defined within respective ones of said pair of dies; said pair of dies each having one of a pair of discharge ports placed on opposite sides of the path of travel of the web along a width thereof and for discharging said coating liquid from said pair of liquid reservoirs onto said web; and coating liquid supply means for supplying said coating liquid to said pair of liquid reservoirs at a specific supply amount per unit time based upon a rate of travel of the web multiplied by a preset coat thickness and a preset coat width to simultaneously apply substantially the same amount of said coating liquid to said opposite sides of the web through said pair of discharge ports to deposit the coating liquid on said web at said preset coat thickness and said preset coat width.
U.S. Pat. No. 5,776,251 discloses a duplex type coating apparatus for applying a coating liquid to a web, such as a a long web made of cloth, plastic film, metal sheet, mesh-form metal sheet and porous metal sheet, comprising: a pair of dies disposed on opposite sides of a carrying path of said web; each of said dies having a liquid reservoir for holding said coating liquid; each of said dies having a tip portion with a discharge port extending in a widthwise direction of the web for discharging said coating liquid onto opposite sides of said web; each of said dies having a coating liquid discharge passage extending from the liquid reservoir to the discharge port; coating liquid supplying means for supplying coating liquid to the liquid reservoirs such that the duplex type coating apparatus applies said coating liquid to both sides of said web by discharging same amounts of said coating liquid from each of said discharge ports onto the web; said discharge ports being disposed opposite one another and each having a web entry side edge part and a web exit side edge part; said web entry side edge parts projecting further toward said web than said web exit side edge parts such that the web exit side edge parts define an exit gap therebetween, the web entry side edge parts define an entry gap therebetween and said exit gap is wider than said entry gap so that the coating liquid is prevented from leaking from between said discharge ports; and the web exit side edge parts each having a curved surface extending from the coating liquid discharge passage to the tip part of each of the dies so that a coating pressure of the coating liquid discharged from the discharge passages increases from a lower level adjacent said web entry side edge parts to a higher level adjacent the web exit side edge parts.
U.S. Pat. No. 6,174,372 discloses a coating apparatus for coating opposing sides of a web, such as an elongated web of cloth, plastic film, metal sheet, glass plate, metal sheet in a net-like shape and a porous metal sheet, traveling in a traveling direction along a transfer paths with a coating solution, comprising: dies installed on opposing sides of said transfer path; said dies each including a reservoir for storing the coating solution inside respective ones of said dies; said dies each having an ejection port with an elongated width along a width direction of the web for ejecting the coating solution; ejection paths for the coating solution communicating substantially the elongated width of the ejection ports with respective ones of said reservoirs; coating solution supplying means for supplying a controlled amount of the coating solution to said reservoirs to effect ejection of the coating solution onto opposing sides of the web by ejecting the coating solution respectively from said ejection ports onto the web; each of said ejection paths having a rotary valve with a rotatable valve element of a substantially cylindrical shape extending parallel to and along a width of the elongated width of the ejection ports which opens said ejection path when positioned within a first angular range and closes said ejection path when positioned within a second angular range; and driving means for pivoting said rotatable valve elements of the rotary valves between said first angular range and said second angular range to provide intermittent flow of said coating solution to apply an intermittent coating to said web.
EP 1 191 622A1 discloses a lithium ion secondary battery separator comprising a sheet (A) with an average film thickness of 10-35 μm, a basis weight of 6-20 g/m2, a gas permeability (JIS P8117) of no longer than 100 seconds, a MacMullin number of 10 or smaller at 25° C. and a MacMullin number×average film thickness value of no greater than 200 μm. EP 1 191 622A1 further discloses that the sheet is composed of fibers and the average diameter of the fibers composing the sheet is ½ to 1/10 of the average film thickness of the sheet (A), wherein the sheet (A) is preferably composed of a polyester, aromatic polyamide, polyphenylene sulfide or polyolefin or a combination of two or more thereof. EP 1 191 622A1 also discloses that the sheet (A) is preferably a nonwoven fabric and that the porous film may also contain a porous inorganic filler if necessary, in addition to the sheet (A) and porous organic polymer film (B).
EP 1 191 622A1 also discloses that the separator preferably comprises a porous film with an average film thickness of 10-35 μm and a basis weight of 10-25 g/m2, which contains a porous organic polymer film (B) that surrounds a sheet (A), and is swelled with the electrolyte solution and retains it, EP 1 191 622A1 discloses that the porous film can be obtained by impregnating and coating the sheet (A) into a solution of the polymer used to form the porous organic polymer film (B), and then removing the solvent. The following methods may be mentioned as specific methods for fabrication of the porous film: 1) a method in which a polymer used to form the porous organic polymer film (B), a solvent which dissolves it and is compatible with water, and a phase separating agent (gelling agent or pore forming agent) are mixed and dissolved together, the sheet (A) is impregnated and coated with the dope solution, and the resulting film is then immersed in an aqueous coagulation bath to coagulate the polymer used to form the porous organic polymer film (B), and finally washed and dried to obtain a porous film; 2) a method in which a polymer used to form the porous organic polymer film (B), a volatile solvent which dissolves it and a plasticizing agent are mixed and dissolved together and the sheet (A) is impregnated and coated with the dope solution and then dried to remove the volatile solvent, after which the plasticizing agent is dissolved and then extracted with a volatile solvent that does not dissolve the polymer used to form the porous organic polymer film (B), and drying is carried out to obtain a porous film; and 3) a method in which a polymer used to form the porous organic polymer film (B) is mixed with a plasticizing agent, the mixture is heated to plasticize and melt the polymer used to form the porous organic polymer film (B) and the sheet (A) is impregnated and coated with this dope, after which the film is cooled to hardness, the plasticizer is dissolved and then extracted with a volatile solvent that does not dissolve the polymer used to form the porous organic polymer film (B), and drying is carried out to obtain a porous film.
EP 1 298 740A discloses a process for production of a composite porous film composed of an organic polymer compound surrounding a porous support, which process comprises coating both sides of a porous support with a solution (dope) of an organic polymer compound and a water-soluble organic solvent using a coating apparatus, subsequently subjecting it to an air gap step and conveying the coated porous support into a coagulating bath containing a coagulating solution consisting of water or a mixture of water with the same solvent as said organic solvent, immersing said porous support in said coagulating bath so that the coated film on both surfaces of said coated porous support contacts directly with the coagulating bath for coagulation, and then washing and drying it. EP 1 298 740A further discloses coating the dope on both sides of the support e.g. applied to supports such as polyolefin fine porous films, wherein it is thought virtually none of the dope is adequately impregnated into the interior, or for fiber-formed two-dimensional sheet supports such as nonwoven fabrics, wherein the dope is adequately impregnated into the interior. EP 1 298 740A also discloses a method involving passing the dope through two opposing dies with a prescribed clearance across the path of conveyance of the support having a narrow clearance between the support and the die lip conveying exit ends of the support conveying exit lips from the die discharge section, and with the dope supplied in a quantitative manner in the widthwise direction accumulates in a liquid pool space formed by positioning opposite the support, allowing continuous impregnation and dispensing of the dope into the support. In this system, the thickness of the coated film can be controlled by changing the clearance between the support and the die lip conveying exit ends at the tips of the lips at the support exit end. This is clearly a process in which the wet layer thickness is determined by the clearance between the support and the die lip. The “air gap step” disclosed in EP 1 298 740A is a step in which the coating apparatus, for example the dispensing/smoothing jig or the die lip, and the coagulating bath are not directly in contact, one of the purposes of which being to provide a leveling effect and hence to realize uniform coating of the dope on both sides of the porous support. The dope compositions, none of which contains a pigment, and coagulating solutions exemplified in EP 1 298 740A are summarized in the Table below:
solvent mixturecoagulation solutionwt % in PVdF(parts by wt)(parts by wt)ExamplecopolymerconcentrationPPG-PPG-nrVDFHFPCTFEPFVEMw[wt %]DMACTPGBD400H2ODMACTPGBD400192.04.53.5—4.1 × 1051264——532——589.58.81.7—6.8 × 105157—3—62.4—1.6—688.74.46.9—5.3 × 10513.56——462——2791.3—3.55.21.0 × 106155——562——2888.74.46.9—5.3 × 10586——462——2992.04.53.5—4.1 × 1051510———62.4—1.6—PFVE = perfluorovinyl etherTPG = tripropylene glycolBD = 1,3-butanediol
JP 10-64503A discloses a separator which is an integrated composite of a nonwoven fabric and an adhesive layer, and a process for its production and describes production of a nonwoven fabric-composited PVdF-based porous film by casting a solution (dope) of PVdF onto a carrier film and then pressing a nonwoven fabric thereover to impregnate the carrier film with the coagulating bath.
A drawback of this approach is that the separators obtained have a poor ionic conductivity. An alternative approach is to use a dope which is a dispersion of a metallic oxide/hydroxide in a solution of the polymer binder to increase the ionic conductivity.
EP 0 232 923A discloses an ion-permeable diaphragm comprising an organic fabric embedded in a film forming mixture of a particulate inorganic hydrophilic material and an organic polymeric binder. EP 0 232 923A further discloses a process of preparing this ion-permeable diaphragm as defined comprising the steps of: i) mixing the particulate inorganic hydrophilic material with a solution of the polymeric binder in an appropriate solvent to form a slurry; ii) uniformly spreading said slurry on an inert flat surface to form a wet sheet; iii) immersing the stretched organic fabric into the wet sheet; iv) removing the solvent by evaporation and/or lixiviation; and v) removing the sheet from the said surface.
WO 93/15529A discloses a method for making a porous membrane with a symmetrical structure which is gas-tight when saturated with electrolyte and whereby, according to this method, a solution is made from an organic binding agent in a solvent and the solvent is removed by means of extraction through immersion in an organic non-solvent, characterized in that an amount of metal oxide and/or metal hydroxide is added to the solution. WO 93/15529A further discloses a membrane made according to this method and an electrochemical cell containing said membrane between two electrodes preferably characterized in that it is an alkaline cell and in that the membrane is saturated with electrolyte and thus forms a separator between two electrodes. WO 93/15529A exemplifies separators based upon polysulphone as the binder and zirconium oxide or zirconium oxide and zinc oxide as the metal oxide or hydroxide without using reinforcing porous polymer supports. Such non-reinforced separators have been commercialized as ZIRFON® separators and exhibit good wettability, low ionic resistance and a high bubble point, but have a typical asymmetric pore structure with finger-like cavities and took 30 minutes to manufacture all of which is unfavourable.
WO 2006/015462A discloses a process for preparing an ion-permeable web-reinforced separator membrane, comprising the steps of: providing a web and a suitable paste, guiding said web in a vertical position, equally coating both sides of said web with said paste to produce a paste coated web, and applying a symmetrical surface pore formation step and a symmetrical coagulation step to said paste coated web to produce a web-reinforced separator membrane. WO 2006/015462A further discloses a web-reinforced separator membrane, characterised in that the web is positioned in the middle of the membrane and both sides of the membrane have the same pore size characteristics and an apparatus for providing a web-reinforced separator membrane, comprising a web-unwinding station for web-tension control, a spreader roller, a coater with double-side coating with double-sided coating system with automatic paste feeding with vertical (guided) web transportation, and guiding rollers in a heated coagulation bath.
In a poster presented by W. Doyen et al. at the World Hydrogen Technologies Convention, held at Montecatini Terme in Italy between 4th and 7 Nov. 2007, discloses the development of an advanced separator in three thicknesses (250, 550 and 950 μm) and in two temperature versions (80° C. and 120° C.) for use in high temperature alkaline water electrolysis, referred to as the “NEW-ZIRFON® separator. The “NEW-ZIRFON® separator is reinforced with a polypropylene, EFTE or PEEK fabric and exhibits permanent hydrophilicity, good wettability in strongly alkaline solutions, low ionic resistance (0.13 Ω·cm2 in 6M KOH at 70° C. for the 550 μm thick version), capability of operating at current densities up to 10 kA/m2, no dimensional changes, a tensile strength of at least 25 MPa, a symmetric pore structure, a total porosity between 50 and 55%, a bubble point above 7 bar and a double skinlayer with identical pores at both sides (mean value 0.08 μm) thereby offering a double safety for preventing the mixing of gases. W. Doyen et al. also discloses that the continuous vertical double-sided coating process disclosed in WO 2006/015462A1 is capable of manufacturing 50 cm wide separators. However, the production technology disclosed in WO 2006/015462A1 does not lend itself to large-scale production of ion-permeable web-reinforced separator membranes.
The smoothness of a separator is extremely necessary to ensure optimal performance. Furthermore, it is desirable to develop manufacturing technology capable of large-scale production of ion-permeable web-reinforced separators.